In traveling-wave tubes a stream of electrons is caused to interact with a propagating electromagnetic wave in a manner which amplifies the electromagnetic energy. In order to achieve such interaction, the electromagnetic wave is propagated along a slow-wave structure, such as a conductive helix wound about the path of the electron stream or a folded waveguide type of structure in which a waveguide is effectively wound back and forth across the path of the electrons. The slow-wave structure provides a path of propagation for the electromagnetic wave which is considerably longer than the axial length of the structure, and hence, the traveling wave may be made to effectively propagate at nearly the velocity of the electron stream. Interaction between the electrons in the stream and the traveling wave causes velocity modulation and bunching of the electrons in the stream. The net result may then be a transfer of energy from the electron stream to the wave traveling along the slow-wave structure.
Since the electron stream is projected along the axis of the tube proximate to the slow-wave structure, the electron stream must be precisely constrained to its axial path in order to prevent excessive impingement of electrons on the slow-wave structure. Generally, this is accomplished by immersing the electron stream in a strong axial magnetic field which tends to provide the required focusing so that the electron stream may pass as closely as possible to the slow-wave structure without excessive interception of electrons by the slow-wave structure. In one of the early techniques for providing the constraining axial magnetic field, the slow-wave structure is aligned coaxially within a long solenoid wound of a conductor carrying a relatively large electrical current. Another early focusing scheme for traveling-wave tubes involves the use of a single large permanent magnet, of a length substantially equal to that of the slow-wave structure, disposed about the slow-wave structure with a pole piece at each end of the magnet. While solenoids and permanent magnets have been able to provide satisfactory focusing, the excessive size and weight of these focusing arrangements have made tubes focused in this manner impractical for many mobile applications.
In order to provide more compact focusing devices for traveling-wave tubes, periodic permanent magnet focusing arrangements were developed in which a plurality of like short annular permanent magnets are disposed in axial alignment along and about the slow-wave structure with a plurality of annular ferromagnetic pole pieces interposed between and abutting adjacent magnets. The magnets are magnetized axially and arranged with like poles of adjacent magnets confronting one another so that there is produced, along the axis of the tube, a periodic magnetic field of sinusoidal distribution, with zero field occurring at each pole piece and with a period equal to twice the pole piece spacing.
The periodic permanent magnet focusing arrangements are terminated at each end by annular ferromagnetic pole pieces to which the electron gun and the electron collector assemblies, respectively, are attached. A fringing magnetic field extends beyond the gun and collector pole pieces and influences the electron flow, usually in an undesirable manner. It is often deemed advisable to exclude this fringing magnetic field from the electron gun region in order to achieve a condition known as Brillouin flow. This has been done by surrounding the electron gun with a ferromagnetic shield and by keeping the aperture in the gun pole piece, through which the electron stream enters the periodic field region, small.
Confined flow focusing in which the cathode is located in a region of a relatively low, but non-vanishing magnetic field, is frequently used in solenoid focused tubes. Confined-flow focusing requires a larger main magnetic field to focus the electron stream than in the case of Brillouin flow. However, in the stronger magnetic field employed with confined-flow focusing, the electrons are less affected by RF defocusing fields and other perturbing effects; hence, improved beam transmission through the slow-wave structure results.
In solenoid confined-flow focusing arrangements, the desired magnetic field in the vicinity of the cathode is provided by leakage of the solenoid-generated focusing field through the aperture in the gun pole piece. The magnitude of the magnetic field at the cathode may be controlled by controlling the diameter of the gun pole piece aperture and the distance of the cathode from this aperture. A larger aperture extends the distance over which the magnetic field monotonically builds up from a small value at the cathode to the much larger uniform value required in the RF interaction region of the tube. With a properly matched convergent flow electron gun of the Pierce type, the magnetic field adds slightly to the reduction of the electron stream diameter obtained by the electrostatic field alone.
In a conventional periodic permanent magnet focusing arrangement wherein the magnets all have substantially the same axial extent, the axial fringing magnetic field does not decrease monotonically as a function of distance from the gun pole piece in the electron gun region, but rather has a reversal in polarity which generally occurs between the cathode and anode of the gun in a region where the electrons are compressed by the electrostatic field. This reversal in the polarity of the magnetic field gives rise to an outward magnetic force on the electrons and, therefore, reduces the beam convergence produced electrostatically by the gun design in the vicinity of the field reversal. Thus, in the past, it has not been practical to achieve confined-flow focusing with a periodic permanent magnet arrangement.
In a few instances where confined-flow focusing has been used in conjunction with periodic permanent magnet focusing, the confined-flow in cathode region has been achieved by coaxially disposing additional permanent magnets and pole pieces about the electron gun. This type of arrangement is shown in FIG. 35, page 46 of Practical Theory and Operation of Traveling Wave Tubes and Microwave Glossary, by Reginald D. Perkins, R&L Technical Publishers, San Jose, Calif., November 1973 (Revised May 1977). Design considerations, as well as both computer simulation and test data for such an arrangement are given in "A Convergent Confined-Flow Focusing System for Millimeter Wave Tubes", by J. R. Legarra et al., Technical Digest IEDM, 1983, pages 137-140, and "RF Beam Spread in PPM Confined-Flow TWTs", by W. R. Ayers et al., Technical Digest IEDM, 1985, pages 357-360.
When permanent magnets and pole pieces are disposed about the electron gun of a traveling-wave tube, a danger of arcing is present because to be effective the magnets need to be close to the high voltage ceramic portion of the gun and, therefore compromise the breakdown spacing. Moreover, some adjustment of the magnetic field is generally necessary during initial operation of the traveling-wave tube, and magnets in this location present a physical hazard. In addition, stray fields from the larger diameter magnets surrounding the gun make location of the traveling-wave tube more critical with respect to other system components.
Additional background of interest with respect to the present invention are the computer simulations described in Hughes Aircraft Company, Electron Dynamics Division, Technical Report No. 100, "PPMODD, PPMEVEN, and STICKUP. Computer Programs for Finite Length PPM Stacks With Magnets Sticking Up Beyond the Pole Pieces", by Jon A. Davis, February 1985. These computer programs served as analytical models of samarium cobalt periodic permanent magnet stacks without any aperture in the end pole pieces, and they also predicted the magnetic field at the cathode. A conclusion reached was that, by making the magnet next to the gun pole piece half as thick as the others, the magnetic field at the cathode is substantially eliminated in the absence of an aperture in the gun pole piece.